Android SurfaceView每次触摸生成新圆并保留历史圆的实现问题

问题原因 #

• 原有代码每次渲染循环都会调用canvas.drawARGB(255,255,255,255)清空画布，之前绘制的圆形会被直接覆盖
• 仅存储了最后一次触摸的坐标，没有维护所有历史圆形的坐标、颜色等属性
• 未给当前View绑定触摸监听器，onTouch回调默认不会触发
• 画笔颜色仅在初始化时生成一次，无法实现每个圆形随机颜色的需求

修改后完整代码 #

import android.graphics.Canvas;
import android.graphics.Paint;
import android.content.Context;
import android.util.AttributeSet;
import android.view.MotionEvent;
import android.view.SurfaceHolder;
import android.view.SurfaceView;
import android.view.View;

import java.util.ArrayList;
import java.util.List;
import java.util.Random;

public class ArtSurface extends SurfaceView implements Runnable, View.OnTouchListener {

    private SurfaceHolder aHolder;
    private Thread aThread;
    private boolean aFlag = false;

    private Paint aPaint;
    private Random cRandom;
    // 存储所有历史圆形的列表
    private List<Circle> circleList = new ArrayList<>();

    // 圆形属性实体类
    private static class Circle {
        float x;
        float y;
        int color;
        float radius = 50;

        public Circle(float x, float y, int color) {
            this.x = x;
            this.y = y;
            this.color = color;
        }
    }

    public int randomColor(){
        if (cRandom == null){
            cRandom = new Random();
        }
        int randomCol = 0xff000000 + 256 * 256 * cRandom.nextInt(256) + 256 * cRandom.nextInt(256) + cRandom.nextInt(256);
        return randomCol;
    }

    public ArtSurface(Context context, AttributeSet attrs) {
        super(context, attrs);
        aHolder = getHolder();
        aPaint = new Paint();
        // 绑定触摸监听
        setOnTouchListener(this);
    }

    public void resume(){
        aThread = new Thread(this);
        aFlag = true;
        aThread.start();
    }

    public void pause(){
        aFlag = false;
    }

    @Override
    public boolean onTouch(View view, MotionEvent motionEvent) {
        switch(motionEvent.getAction()){
            case MotionEvent.ACTION_DOWN:
                // 每次触摸新增一个随机颜色的圆形存入列表
                circleList.add(new Circle(motionEvent.getX(), motionEvent.getY(), randomColor()));
                break;
            case MotionEvent.ACTION_UP:
                break;
        }
        return true;
    }

    @Override
    public void run() {
        while(aFlag){
            if(!aHolder.getSurface().isValid())
                continue;
            Canvas canvas = aHolder.lockCanvas();
            // 清屏
            canvas.drawARGB(255,255,255,255);
            // 遍历绘制所有历史圆形
            for (Circle circle : circleList) {
                aPaint.setColor(circle.color);
                canvas.drawCircle(circle.x, circle.y, circle.radius, aPaint);
            }
            aHolder.unlockCanvasAndPost(canvas);
            // 加16ms延时控制帧率在60左右，降低CPU占用
            try {
                Thread.sleep(16);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}

内容的提问来源于stack exchange，提问作者Muiruri